This is because you do not have the SMTP_Auth option enabled in your email reader. In Microsoft readers like Outlook and Outlook Express, this option is called: My Server Requires Authentication, or Outgoing Mail Server (SMTP) requires authentication.
- Go to Tools > Email Accounts
- Select the Account with the trouble and click "Change"
- Click on More Settings
- Click on Outgoing Server Tab
- Click "Outgoing Mail Server (SMTP) requires authentication" and make sure "Use same settings as incoming mail server" is selected (see attached image)
For Outlook Express:
- Go to Tools > Accounts
- Select the problem account from the list and click "Properties"
- Click on the servers tab
- Check the box "My Server Requires authentication"
- Click OK.
- Tools > account Settings
- At the bottom of the list of options on the left is "Outgoing Server (SMTP)" Select this option and then "Edit" the problematic account.
- Check the box under "Security and Authentication" called "Username and password"
- type in the full email address in the username box.